Surface insulation for magnetic sheet steel



Patented Sept. 17, 1940 UNITED STATES PATENT OFFICE SURFACE INSULATION FOR MAGNETIC SHEET STEEL Weston Morrill, Pittsfield, Mass., assignor to General Electric Company, a corporation of New The present invention relates to the use of insulated coatings on sheet steel and more particularly to the use of very thin inorganic coatings adapted for use on transformer laminations.

Prior to the present invention, it has been customary in the construction of laminated transformer cores to coat thesheet steel laminations after anneal with a core plate enamel to provide a .suitable space relation between the laminated sheets.

It is one of the objects of the presentinvention to provide an inexpensive insulation coating on sheet steel which is suitable for use on transformer laminations and may be applied to the laminations before annealing. It is also an object of my invention to provide an insulating coating which not only has the ability to so insulate the surface of the steel that the use of a core plate enamel may be eliminated but also 0 the ability to prevent the laminations from sticking together during the annealing process.

In carrying out my invention, I mix about 20 parts by weight of expanded vermiculite, which is an aluminum-magnesium silicate sold under 25 the trade name Zonolite, with about 100 parts by weight of water. The expanded material is obtained by introducing crude vermiculite into an expansion furnace heated at about 2000 F. This results in the production of a honeycombed 30 brittle granule which immediate cooling toughens into a durable aggregate in a manner well known inthe art. The mixing of the water and expanded vermiculite is effected by grinding the vermiculite and water mixture until the ver- 35 miculite is in an extremely finely divided state producing a suspension of vermiculite in water and having about the consistency of house paint. This condition may be obtained by ball milling the water vermiculite mixture for about 20 to 24,

40 hours, the vermiculite initially being capable of passing through a 60 mesh screen. The particles of vermiculite after mixing in the ball mill are in the form of small leaves or fiat crystals.

The water-vermiculite mixture thus prepared 45 is in effect a paint in which water is the vehicle instead of oil. It may be applied to sheet steel by means of a brush, spray, or rubber rolls and then dried in an oven at a temperature suinciently high to evaporate the water. The coated 50 sheets may be annealed in the usual manner, i. e.,

in a pack in a furnace having a roof temperature of about 875 C. and an atmosphere of air or neutral gas. The water vermiculite coating applied to silicon steel laminations as heretofore 55 pointed out provides a high degree of insulation between successive laminations andappears to give a relatively small reduction in total watt loss, i. e., up to about 2.5% as compared with laminated sheets having a core plate enamel coating. This reduction in watt loss is possibly due to the fact-that the coating prevents excessive oxidation of the sheet steel. Furthermore, the coating prevents sticking together of the steel sheets during the annealing process. The vermiculite forms a very tight coating which cannot be rubbed off the steel or removed any easier than the regular anneal scale and gives a surface insulation resistance which is greater in both air and hot oil than that which can be obtained with core plate enamel heretofore employed in transformer construction.

The vermiculite insulation forms a very thin film coating and causes no appreciable increase in the space factor permissible in transformer construction. The vermiculite, during the annealing process, apparently combines with the natural scale formation on the steel thereby providing a highly insulating coat. vermiculite, however, even without the assistance of the steel scale is a particularly efilcient insulator due to its leafing action.

While I prefer to employ a mixture of about 20 parts vermiculite and about 100 parts of water, these quantities are not critical and may be varied widely. Also, instead of employing water, I may mix the vermiculite with Water solutions which may be beneficial to the steel. Although I prefer to employ a mixture of vermiculite and water, I may employ instead of the vermiculite other materials having approximately the same chemical analysis or crystallographic form.

What I claim as new. and desire to secure by Letters Patent of the United States, is:

l. The method of electrically insulating sheet steel which comprises applying to the steel a film consisting of a mixture of water and expanded vermiculite, drying the mixture to evaporatethe water and baking the vermiculite onto the steel during an anneal.

2. In the'manfacture of magnetic sheet steel, 5 the process which comprises coating said steel sheets prior to annealing with an electrically insulating film consisting of a water-vermiculite mixture.

3. Sheet steel suitable for use as a transformer lamination and having an insulating coating thereon, said coating consisting substantially of a vermiculite film baked onto said sheet steel.

Silicon steel sheets suitable for use in transformer laminations, said sheets having an electrically insulating film thereon, said film consisting substantially of vermiculite baked thereon.

5. Annealed silicon steel having a coating thereon, said coating consisting of vermiculite bonded to the natural scale formation of the steel.

6. Annealed silicon steel having a coating thereon, said coating consisting of vermiculite bonded to the natural scale formation of the steel, said coating being free from alkaline bonding ma- 10 terial.

7. Siliconsteel having a thin, tight, insulating coating thereon, said coating consisting substantially of vermiculitebonded to said steel and having the property of preventing sticking and excessive oxidation during anneal and causing no appreciable increase in the space factor when the coated steel is employed as laminations in transformer cores.

WESTON MQRRILL. 10 

